ZnO Nanostructure-Based Intracellular Sensor.

Recently ZnO has attracted much interest because of its usefulness for intracellular measurements of biochemical species by using its semiconducting, electrochemical, catalytic properties and for being biosafe and biocompatible. ZnO thus has a wide range of applications in optoelectronics, intracellular nanosensors, transducers, energy conversion and medical sciences. This review relates specifically to intracellular electrochemical (glucose and free metal ion) biosensors based on functionalized zinc oxide nanowires/nanorods. For intracellular measurements, the ZnO nanowires/nanorods were grown on the tip of a borosilicate glass capillary (0.7 µm in diameter) and functionalized with membranes or enzymes to produce intracellular selective metal ion or glucose sensors. Successful intracellular measurements were carried out using ZnO nanowires/nanorods grown on small tips for glucose and free metal ions using two types of cells, human fat cells and frog oocytes. The sensors in this study were used to detect real-time changes of metal ions and glucose across human fat cells and frog cells using changes in the electrochemical potential at the interface of the intracellular micro-environment. Such devices are helpful in explaining various intracellular processes involving ions and glucose.

Intracellular K+ concentration decrease is not obligatory for apoptosis.

K(+) efflux is observed as an early event in the apoptotic process in various cell types. Loss of intracellular K(+) and subsequent reduction in ionic strength are suggested to release the inhibition of proapoptotic caspases. In this work, a new K(+)-specific microelectrode was used to study possible alterations in intracellular K(+) in Xenopus laevis oocytes during chemically induced apoptosis. The accuracy of the microelectrode to detect changes in intracellular K(+) was verified with parallel electrophysiological measurements. In concordance with previous studies on other cell types, apoptotic stimuli reduced the intracellular K(+) concentration in Xenopus oocytes and increased caspase-3 activity. The reduction in intracellular K(+) was prevented by dense expression of voltage-gated K (Kv) channels. Despite this, the caspase-3 activity was increased similarly in Kv channel-expressing oocytes as in oocytes not expressing Kv channels. Thus, in Xenopus oocytes caspase-3 activity is not dependent on the intracellular concentration of K(+).

In Situ Quantification of Glucose Concentration in Airway Surface Liquid With Functionalized ZnO Nanorod-Coated Microelectrodes.

The surface of the airways that conduct gases into and out of the lungs has components that protect the host from inhaled and aspirated pathogens. The thin (4-7 µm height) layer of airway surface liquid (ASL) that lines the airways has physicochemical properties that are important for normal function of these antimicrobial components. Among these properties, low glucose concentration is required for normal antimicrobial activity. Current methods for assessing the ASL have important flaws (temporal resolution, dilution factors, collection volume), which have been a recurring obstacle for understanding diseases in which ASL composition is abnormal. To circumvent these problems, microelectrodes coated with ZnO nanorods and immobilized glucose oxidase was used to determine glucose concentration in ASL of well-differentiated cultures of human airway epithelia. The sensor responded to glucose linearly over a concentration range of 0.128 to 8 mM and the effects of electroactive interferents were minimal. The measured concentration of glucose in ASL was consistent with values previously reported. This method confirms the presence of a transepithelial glucose concentration gradient in human airway epithelia and is an important step towards characterizing the physicochemical properties of ASL and understanding diseases caused by changes in ASL composition.

Functionalized ZnO nanorod-based selective magnesium ion sensor for intracellular measurements.

ZnO nanorods were grown on a silver-coated tip of a borosilicate glass capillary (0.7 µm in tip diameter) and used as selective potentiometric sensor of intracellular free Mg(2+). To functionalize the ZnO nanorods for selectivity of Mg(2+), a polymeric membrane with Mg(2+)-selective ionophores were coated on the surface of the ZnO nanorods. These functionalized ZnO nanorods exhibited a Mg(2+)-dependent electrochemical potential difference versus an Ag/AgCl reference microelectrode within the concentration range from 500 nM to 100 mM. Two types of cells, human adipocytes and frog oocytes, were used for the intracellular Mg(2+) measurements. The intracellular concentration of free Mg(2+) in human adipocytes and frog oocytes were 0.4-0.5 and 0.8-0.9 mM, respectively. Such type of nanoelectrode device paves the way to enable analytical measurements in single living cells and to sense other bio-chemical species at the intracellular level.

Growth and Structure of ZnO Nanorods on a Sub-Micrometer Glass Pipette and Their Application as Intracellular Potentiometric Selective Ion Sensors.

This paper presents the growth and structure of ZnO nanorods on a sub-micrometer glass pipette and their application as an intracellular selective ion sensor. Highly oriented, vertical and aligned ZnO nanorods were grown on the tip of a borosilicate glass capillary (0.7 µm in diameter) by the low temperature aqueous chemical growth (ACG) technique. The relatively large surface-to-volume ratio of ZnO nanorods makes them attractive for electrochemical sensing. Transmission electron microscopy studies show that ZnO nanorods are single crystals and grow along the crystal's c-axis. The ZnO nanorods were functionalized with a polymeric membrane for selective intracellular measurements of Na⁺. The membrane-coated ZnO nanorods exhibited a Na⁺-dependent electrochemical potential difference versus an Ag/AgCl reference micro-electrode within a wide concentration range from 0.5 mM to 100 mM. The fabrication of functionalized ZnO nanorods paves the way to sense a wide range of biochemical species at the intracellular level.

Functionalised ZnO-nanorod-based selective electrochemical sensor for intracellular glucose.

In this article, we report a functionalised ZnO-nanorod-based selective electrochemical sensor for intracellular glucose. To adjust the sensor for intracellular glucose measurements, we grew hexagonal ZnO nanorods on the tip of a silver-covered borosilicate glass capillary (0.7 microm diameter) and coated them with the enzyme glucose oxidase. The enzyme-coated ZnO nanorods exhibited a glucose-dependent electrochemical potential difference versus an Ag/AgCl reference microelectrode. The potential difference was linear over the concentration range of interest (0.5-1000 microM). The measured glucose concentration in human adipocytes or frog oocytes using our ZnO-nanorod sensor was consistent with values of glucose concentration reported in the literature; furthermore, the sensor was able to show that insulin increased the intracellular glucose concentration. This nanoelectrode device demonstrates a simple technique to measure intracellular glucose concentration.